1. Field of the Invention
This invention relates to the production of nickel-chromium-iron base alloys and members thereof produced by working and suitable heat treatment to provide solution hardening, that are particularly useful at elevated temperatures of up to 1325.degree. F. Such alloys and members thereof have particular utility in nuclear reactors employing sodium in that these members have a very low corrosion rate and also have a low swelling when exposed to intense radiation for the desired life of such members under such radiation and sodium contacting conditions.
2. Description of the Prior Art
Nickel-chromium-iron base alloys are broadly well known in the art and a description of many of these alloys is set forth in "Metals Handbook", Volume 2, 8th Edition published in 1964 by the American Society for Metals, particular references being had to the chapter entitled "Heat Treating of Stainless Steel and Heat Resisting Alloys" beginning on page 243 and ending on page 267. The section headed "Heat Treating of Stainless Steel" which extends from page 243 to page 254, lists the properties and characteristics of austenitic stainless steels including the heat treatment, resistance to oxygen corrosion, the discussion of the Sigma phase and its possible impact on the alloy characteristics, as well as the mechanical properties of the various alloys obtained by particular heat treatments. An extended description of a wide class of heat resisting alloys is contained in the section entitled "Heat Treating of Heat-Resisting Alloys" extending from page 257 to page 267 of this "Metals Handbook" citation.
The alloy composition of the prior art that approaches most closely the compositions of the present invention is alloy designated HT, set forth on page 258 of the Metals Handbook, which HT alloy comprises 15% chromium, 35% nickel, 0.55% carbon and the balance being iron. This alloy is described as a casting alloy and obviously is not intended to be cold or hot worked or otherwise shaped into a member but is simply employed as a casting which, at most, is finished by grinding or machining of the casting to a desired surface finish or shape without modifying the grain fiber, texture or shaped by substantially deformation as is effected by appreciable hot and/or cold working.
U.S. Pat. No. 2,857,266 refers to the HT alloy and is directed to an improvement thereof which patent alloy comprises between 23 to 27% chromium, between 34 and 37% nickel, between 1 to 2% molybdenum, between 1 and 21/2% silicon, between 1 and 4% manganese, between 0.05% and 0.40% aluminum, nitrogen between 0.15% and 0.40%, carbon between 0.45 and 0.55%, boron between 0.001% and 0.04%, with impurities not exceeding 0.4% of phosphorus and 0.04% of sulfur, and the balance being iron. This patent discloses the various physical properties of the alloys particularly at tempertures of 1800.degree. F. and higher. Nothing is said in this patent about shaping or working of the alloy and it is quite evident that the alloy is cast into shape without any hot or cold working.
U.S. Pat. No. 3,796,567 is directed to nickel-chromium-iron base alloys incorporating small amounts of various additional components such as silicon, molybdenum, titanium and manganese, in which the chromium may vary from 16 to 25% and the nickel from 8 to 25% by weight. The patent is specifically directed to the deoxidation of these alloys by adding metallic calcium in order to permit welding without globule formation.
U.S. Pat. No. 3,235,378, discloses in column 5, nominal compositions of a variety of nickel-chromium base stainless steels, as well as certain high nickel base alloys containing substantial amounts of chromium. This patent compares such stainless steels on the basis of their oxidative and sulfide corrosion at high temperatures under conditions which are prevalent in gas turbines for example.
At the present time, liquid metal fast breeder reactors (LMFBR) are an important development in the energy field. These reactors employ liquid sodium as the medium for absorbing energy from nuclear fuel elements. In service, the liquid sodium is at temperatures of from about 1000.degree. F. to 1200.degree. F., with occasional hot spots reaching 1325.degree. F. While the liquid sodium employed in these reactors is free from oxygen or water to the lowest possible reasonable extent in order to avoid the formation of sodium oxides which are extremely corrosive to many metals, molten sodium itself is a corrosive material and many of the metallic elements dissolve therein to some extent. Thus iron, nickel and chromium, are soluble to some modest degree in hot sodium. Since the molten sodium flows from the highest temperature areas in the vicinity of the fuel elements to a heat exchanger where its temperature drops substantially, any dissolved metallic elements that have a higher solubility in sodium at the elevated temperatures will reach a low temperature point where, because of lower solubility, a portion of the dissolved elements may precipitate out upon the surfaces of the heat exchanger and, consequently, a transfer thereto will take place of metallic elements from the nuclear fuel portions of the reactor. Undesirable results occur in that metal is removed from the fuel elements and in-core components and precipitated on heat exchanger surfaces. Reference should be had to "Symposium on Chemical Aspects of Corrosion and Mass Transfer AIMG-- 1971", and particularly the paper entitled "Sodium Corrosion Behavior of Alloys For Fast Reactor Applications" by G. A. Whitlow et al, wherein is listed the corrosion characteristics of some of the metals which would be present in alloy members being considered for the Liquid Metal Fast Breeder Reactor (LMFBR) at the present time.
The well known 316 stainless steel has been proposed for use in the LMFBR system for components such as fuel cladding, ducts, grid supports and structural components. At high irradiation of the order of 10.sup.23 nvt., it has been found that 316 stainless steel and similar alloys will swell very substantially. Due to such swelling, components comprising such as 316 stainless steel will bow, bend or distort substantially, usually in erratic and unpredictable directions. Consequently, unless ample spacing provision is made, channels for flow of liquid sodium and around the fuel elements in particular, may be greatly restricted with the result that heat produced by the fuel elements is not properly or uniformly absorbed by the flowing sodium. Hot spots will occur under these conditions with gross overheating possible so that failure of such mechanical components and nuclear fuel grids and supports may result. In order to provide an adequate space for flow of molten sodium between or through ducts, fuel elements, grid supports and the like, excessively generous clearances must be present to accommodate reasonably expected swelling with its resulting bowing and distortion so that adequate flow spaces are present at all times within the expected life time of such reactor components.
One of the prime characteristics of a breeder reactor is the operating time required for such reactor to double the amount of fissile nuclear material produced therein as compared to the original quantity of fissile nuclear material present therein. The desired reasonably optimistic doubling time for present day breeder reactor design, assuming the availability of optimum materials, is about 10 years.
The doubling time of a breeder reactor is critically dependent on the spacing of the fuel rods with respect to each other and to a blanket of fertile nuclear material. If the expected swelling is, for example, 25%, the required spacing between fuel elements and so forth to accommodate bowing and distortion is so great that the doubling time of the breeder reactor would be of the order of 30 to 40 years or more. If the expected swelling of the alloy is of the order of 5%, the spacing of the fuel elements and other components is correspondingly reduced and the doubling life may be of the order of 15 to 10 years. Obviously it is of the highest importance to be able to space the fuel elements as closely together as safety and technological considerations permit to secure the shortest doubling time. The efficiency of the reactor and the doubling time are improved very significantly by the closest possible spacing of the fuel rods and other components of the reactor, taking into account all the distortion that may reasonably be expected to occur in service for the expected years of operation of each component.
A second critical factor to be taken into consideration in designing the components for an LMFBR is the rate of corrosion of the metal surface by the hot sodium flowing therepast. High nickel alloys in particular react with the molten sodium at a substantial rate, the corrosion being greater as the high nickel content increases; for example, nickel base alloys of 70% nickel content will corrode at the rate of 5 mils per year or greater on all surfaces exposed to the molten sodium at temperatures of 1325.degree. F. Since it is desirable that a fuel element for instance, has a life of the order of 3 years in a LMFBR, the thickness of cladding of the fuel elements for example, must be originally sufficiently heavy to compensate for the fact that 15 mils or more of the thickness thereof on all surfaces exposed to the sodium will have been eroded away in 3 years service, yet leaving a sufficient metal wall thickness to withstand all of the pressures and stresses to be reasonably expected throughout this period of time. On the other hand, any excess thickness of cladding, for instance, not only increases the cost of the fuel element, but also increases the spacing between fuel elements and other support members and further introduces increased neutron capture losses.
It is therefore quite apparent that it is desirable to have an alloy that exhibits both low swelling under the intense radiation conditions to be expected over the reasonable life of a member of the alloy in the LMFBR and also low corrosion when in contact with sodium over such a prolonged period.